This invention relates to a synthetic semi-cordierite grog intended for use in making refractory bodies, such as insulating bricks, kiln cartop blocks, castables, plastic refractories and the like.
Cordierite is a rarely occurring natural magnesium-aluminum-silicate mineral which can impart good thermal shock resistance to refractory bodies containing the cordierite due to its low thermal expansion characteristics. Stoichiometric cordierite has a theoretical formula 2MgO.2Al.sub.2 O.sub.3.5SiO and a composition of 13.8% MgO, 34.8% Al.sub.2 O.sub.3 and 51.4% SiO.sub.2.
The natural deposits are too impure and scarce to be a valuable commercial source of cordierite. Synthetic cordierites have been produced by sintering mixtures of talc and clay or talc, clay and alumina to form stoichiometric or near stoichiometric cordierite, but complete reaction is difficult without fusing the mass. Synthetic cordierite has also been formed by intimately blending magnesia, alumina and silica in precise proportions and firing to a sintering temperature sufficient to convert these oxides into cordierite. Again, complete reaction is virtually impossible without complete fusing.
Most of the prior art methods used to make cordierite are concerned with producing stoichiometric or near stoichiometric cordierite. Unfortunately, stoichiometric cordierite has a very short firing range, especially where naturally occurring impurities exist. This creates a problem, in that very exact temperature control is necessary to sinter the mixed ingredients to form synthetic stoichiometric cordierite. If the temperature is too low, cordierite will not form; if the temperature is a little too high, it will melt. As can be seen from a standard MgO--Al.sub.2 O.sub.3 --SiO.sub.2 phase diagram, cordierite lies in the mullite field, indicating its instability. Accordingly, the present methods of making cordierite require very precise control to assure that cordierite will be formed, rather than other magnesia-alumina-silica phases.
Because of the limited firing range of stoichiometric cordierite, synthetic stoichiometric cordierite is often formed by fusing (i.e., forming by melting) magnesia, alumina and silica containing ingredients. Fusion increases the amount of stoichiometric cordierite formed from the oxides, compared to sintering. However, fusion is expensive and is not energy efficient because more energy is required to fuse than to sinter, and because of the inefficiency of radiant energy transfer into a molten mass.
The following patents are believed to represent the closest prior art and disclose methods of making synthetic cordierite or compositions having percentages of magnesia, alumina and silica approximating those of cordierite:
U.S. Pat. No. 2,036,190 of Benner et al. discloses a process of making ceramic articles from certain alumino-silicates by melting or by vitrifying and then devitrifying a mixture of the ingredients comprising the ceramic material. Preferably, the ingredients are prefused. The oxides of magnesium, barium, zinc, calcium and gallium may be present in a composition along with alumina and/or silica. This patent does not teach the formation of a semi-cordierite by sintering olivine with the natural minerals referred to hereinafter.
U.S. Pat. No. 2,731,355 of Skinner discloses a method of making synthetic cordierite using natural minerals as starting materials. A natural magnesium-containing mineral such as olivine, serpentine, talc or brucite, is mixed with aluminous material, such as kaolin, bauxite or siliceous bauxite, and silica, if necessary. The magnesium-containing minerals are mixed and melted to produce a composition corresponding to that of forsterite plus enstatitite. Sufficient iron is included in the melt to increase the specific gravity of any ferrosilicon which may be formed during the melting of the raw materials to facilitate its separation from the molten mass by precipitation. The aluminous material and silica are added to the molten magnesium silicate at a temperature of at least 1475.degree. C. in proportions sufficient to form a magnesium-aluminum-silicate melt having the chemical composition of cordierite. The iron and associated ferro-silicon formed during melting is removed, followed by cooling. A slow cooling process is used to develop large cordierite crystals. The fused batch must be cooled to 800.degree. C. in not less than 8 hours and to room temperature in not less than 4 additional hours. The composition produced by this method contains 10-30% MgO, 15-50% Al.sub.2 O.sub.3 and 35-70% SiO.sub.2.
The patented process is a fusion process, not a sintering process. Moreover, the patented method produces a synthetic cordierite from which substantially all iron is removed. The non-stoichiometric cordierite produced in accordance with the present invention advantageously contains a significant amount of iron.
U.S. Pat. No. 2,864,919 of Stringfellow discloses a ceramic composition containing more than 50% of cordierite crystals having a composition of 5-15% MgO, 22-58% Al.sub.2 O.sub.3 and 38-63% SiO.sub.2. The composition is formed by firing a mixture of talc, kaolin and alumina at temperatures from about 2300.degree. to 2700.degree. F. There is no mention of olivine.
U.S. Pat. No. 3,885,977 of Lachman et al. discloses a method of producing a cordierite having a particular orientation of crystals by orienting the precursor materials in the proper direction during the forming process. This may be done only by selecting raw materials which lend themselves to the orientation process. In particular, raw materials which are in the form of flat, planar (platelet) particles, rather than large isodimensional particles are most useful. The raw materials include high purity clay, talc, silica, alumina, aluminum hydroxides and magnesia-yielding chemicals in amounts so that the final composition comprises 9-20% MgO, 30-50% Al.sub.2 O.sub.3 and 41-56.3% SiO.sub.2. The particularly oriented precursor materials are fired at a temperature of 1340.degree.-1350.degree. C. The present invention is not concerned with the particular orienting process disclosed in this patent.